The present invention relates to a method for driving a Plasma Display Panel (PDP), one of the flat display devices, and more particularly, to improvement of the brightness and contrast of a 2-electrode or 3-electrode AC-type PDP.
As shown in FIG. 1, a conventional 3-electrode surface discharge Plasma Display Panel comprises the following elements: scanning electrodes 3 to which a scanning pulse is applied during an address period, common electrodes 4 to which a sustaining pulse 8 is applied for the sustaining of discharge, and data electrodes 2 to which a data pulse 12 is applied for generating a sustaining discharge between the scanning electrode 3 and the common electrode 4 of a selection line.
A cell 5 is formed at an intersection where a vertical electrode comprising a set of the scanning electrode 3 and the common electrode 4, and a horizontal electrode comprising the data electrode 2 cross. The cells are accumulated, and they form one plasma display panel 1.
In addition, referring to FIG. 5, a conventional timing diagram comprises: a data pulse 12 maintaining regular intervals applied to a data electrode 2 as shown in (e) of FIG. 5; a Z-sustaining pulse 8 applied to a common electrode 4 as shown in (a) of FIG. 5; and, a Y-sustaining pulse 9 applied to a scanning electrode 3 as shown in (b), (c) and (d) of FIG. 5, wherein the scanning pulse 10 between Y-sustaining pulses 9 is applied sequentially from a first horizontal electrode S.sub.1 to a horizontal electrode Sm at point m. Moreover, a scanning pulse 10 is applied to the scanning electrode 3, and thereafter an erasing pulse 11 is applied to the scanning electrode 3 at some intervals.
The above-described PDP generates a discharge by a voltage being applied between the vertical and horizontal electrodes of the cell 5 forming a pixel, sustains the discharge by applying a voltage to a horizontal electrode, and regulates the quantity of light generated by changing the length of discharge time within the cell 5.
To show the entire screen, the data pulse 12 for inputting a digital video signal is applied to the data electrode 2 of each cell; the scanning pulse 10 for scanning, the Y-sustaining pulse 9 for sustaining the discharge, and the erasing pulse 11 for terminating the discharge of the cells are applied to the scanning electrode 3 of each cell; and the Z-sustaining pulse 8 for sustaining the discharge is applied to the common electrode 4.
Each pulse indicated above is applied in a matrix form to the horizontal electrode (scanning electrode+common electrode) and the vertical electrode (data electrode) to show the entire screen.
The gradational gray level required to display an image is materialized by setting a difference in the length of discharge time by each cell within the span of time necessary for the showing of the entire image (in the case of NTSC TV, it requires 1/30 seconds). In the case of a flat display device for a MD TV with the capacity of a 1280.times.1024 resolution, a video digital signal required to show an image maintaining a 256 gray level is 8 bits.
FIG. 2 shows the scanning method of a conventional art comprising eight sub fields out of one field for the materialization of a 256 gray scale with an 8-bit digital video signal. In other words, one field comprises a plurality of subfields, and to show images containing the gradational gray level, each subfield is arranged to have a different time for the emission of light.
In FIG. 2, one field comprises eight subfields, each has a Ts time, with a gray level of 2.sup.n =256(n=8). In addition, each subfield has a different emitting time for different lights of T, T/2, T/4, T/8, T/16, T/32, T/64, T/128 and T/256. By adjusting the time for the emission of the light through the eight bit combination and by using the integral effect of eyes for the light, the 256 gray scale is materialized.
According to the pulse timing diagram of the conventional art as shown in FIG. 5, the common electrode 4 between Cl-Cm is applied with the Z-sustaining pulse 8, while applying the Y-sustaining pulse 9 of the same cycle to the scanning electrode 3 between S1-Sm ; however, the timing is different from that of the common electrode.
The scanning pulse 10 and the erasing pulse I 1 are also applied to each scanning electrode 3. The data pulse 12 is applied to the data electrode 2 between D1-Dn at the same timing of the scanning pulse being applied to the scanning electrode. For the radiation of the cell 5 where the scanning electrode 3 and the data electrode 2 cross, the data pulse 12 synchronized to the scanning pulse 10 to be applied to the scanning electrode 3 must be provided to the data electrode 2.
Accordingly, the cell 5 starts to discharge, and the discharge can be sustained by the Z-sustaining pulse 8 and the Y-sustaining pulse 9 being provided to the common electrode 4 and scanning electrode 3. The discharge is terminated by the erasing pulse 11.
For displaying the entire image as it was viewed above, the gray level and contrast of the PDP should be materialized by setting a different length of discharge time of each cell 5 within a fixed time. At this time, the brightness of the image is decided by the gray level shown at the time of driving each cell 5 for the longest span of time. To increase the brightness of the image, the driving circuit of the cell 5 should be so designed as to sustain the maximum length of time for the discharging of the cell 5 within the span of a given time to form a screen.
According to a conventional subfield method, it has to collect digital video signals separately from Most Significant Bit (MSB) to Least Significant Bit (LSB), then form the subfields by assigning the MSB to the discharge time T, and by allocating each bit to the discharge time T/2, T/4, . . ., T/128, respectively, in the order of bits close to the MSB, thus forming the 256 gray scale by using the integral effect of eyes toward the light being emitted from each subfield.
Since the conventional PDP has to be driven by a matrix method, there is a restrictive problem that the data pulses of one or more horizontal electrodes at a time cannot be applied to a given vertical electrode. Because of this reason, the horizontal electrodes have to be driven at different times from each other. Therefore, to form each subfield, time is needed to scan all horizontal electrodes, and the time required for the scanning is increased as the number of the horizontal electrodes increases.
Since the horizontal electrodes are required to be driven at different times from each other, the time being used for the discharging of each cell 5 is reduced as the time of scanning is extended, and it causes the dropping of the brightness and contrast of the PDP.
FIG. 3 shows the scanning of each horizontal electrode toward a time axis according to the subfield method of the conventional art. The subfield can start the scanning of other subfields after terminating the scanning of all horizontal electrodes of a subfield from the restrictive point of the matrix method. As shown in FIG. 4, if the subfield method of the conventional art connects two subfields to reduce the time T.sub.B which emit no light to improve the efficiency of light emission, it requires applying the scanning pulse 10 to a plurality of horizontal electrodes simultaneously at the point, such as a or b, at the same time axis to drive the data pulse 12 being applied to a vertical electrode; however, there is a problem that it is impossible because of a characteristic of the matrix driving method.